The present invention relates to a heat-radiating sheet for dissipating heat generated from an exothermic body, and a heat-radiating structure equipped with said heat-radiating sheet.
Electronic components and other exothermic bodies present in electronic devices as exemplified by cellular telephones have come to be installed at increasingly high densities accompanying the reduced size and thinner design of these devices, while the thermal capacity of these electronic devices is decreasing. Moreover, the degree of sealing of electronic devices is also increasing accompanying improvements in waterproofing and dust-proofing of these devices. Consequently, in addition to the heat generated from exothermic bodies tending to be easily accumulated locally within electronic devices, the temperature of the case surface of electronic devices rises easily. As a result, together with exothermic bodies being susceptible to damage due to accumulation of heat, there is also greater likelihood of causing discomfort for users of such electronic devices due to the local rise in case surface temperature attributable to local accumulation of heat within such electronic devices. Consequently, it is necessary to avoid the local accumulation of heat within electronic devices, and the diffusion of heat generated from exothermic bodies is becoming an important issue.
In the past, graphite sheets were used to efficiently diffuse generated heat in confined, narrow spaces. A graphite sheet is provided on an exothermic body, and the generated heat is forced to diffuse in a direction parallel to the surface of the graphite sheet. However, graphite sheets have the problems indicated below.
(1) Since graphite sheets have a rough surface as well as large rigidity, adequate adhesion is unable to be obtained with an exothermic body even in the case where a large load is applied so as to adhere to the exothermic body. As a result, there are cases in which contact thermal resistance with the exothermic body cannot be adequately lowered. In addition, there are cases in which the application of a large load to the graphite sheet on an exothermic body has a detrimental effect on the exothermic body.(2) The coefficient of thermal conductivity in the direction of thickness of graphite sheets can be enhanced by, for example, changing the crystal structure of the graphite within the graphite sheet in order to allow more efficient transfer of heat from an exothermic body to the graphite sheet. In this case, however, since the coefficient of thermal conductivity in a direction parallel to the surface of the graphite sheet decreases, the heat-diffusing effects of the graphite sheet are diminished.(3) Since graphite sheets have satisfactory electrical conductivity, graphite sheets require insulating measures in cases in which it is preferable that the graphite sheet not demonstrate electrical conductivity.(4) Since graphite sheets lack tackiness, the use of an adhesive is required to attach graphite sheets to an exothermic body. Consequently, together with the attachment of the graphite sheet to an exothermic body being bothersome, the heat-radiating effects of the graphite sheet may decrease or become unstable due to the thickness and thermal conductivity of the adhesive layer provided between the exothermic body and graphite sheet. Moreover, after a graphite sheet has been attached to an exothermic body using an adhesive, when the graphite sheet is reattached after having separated from the exothermic body for reasons such as correcting the attached location, interlayer separation occurs between the graphite layers laminated within the graphite sheet making the graphite sheet susceptible to damage. Consequently, graphite sheets have low reworkability (ease of reattachment).
Japanese Laid-Open Patent Publication No. 2003-168882 discloses a graphite sheet having high rigidity and a heat-radiating sheet equipped with a flexible polymer material layer provided on said graphite sheet in order to resolve the aforementioned problems of graphite sheets. This heat-radiating sheet aims to overcome the aforementioned problems of graphite sheets with a polymer material layer. However, the heat-radiating sheet of Japanese Laid-Open Patent Publication No. 2003-168882 is unable to adequately overcome the aforementioned problems and still has room for improvement.